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Colourful aquarium fish known as swordtails, size up competitors' tailfins before fighting, US scientists show.

Often a male just has to show his big elongated tail, which looks like a pointy sword, to scare off challengers, according to a new study in the latest issue of the journal Animal Behaviour.

Yet another study in the same publication concludes that female swordtails prefer male swordtails with big, striped swords.

Researchers even think the female fixation on these bright appendages led to their emergence in the first place, since female fondness for novel traits and bright colours appears to have preceded the appearance of male swords.

The finding suggests the creation of some secondary sexual characteristics may be influenced, and even controlled, by the opposite sex. For male swordtails, this characteristic is an extension of the tail or caudal fin.

"It is not terribly rigid and cannot be manipulated very efficiently," says Dr Kari Benson, who co-authored the first paper with Dr Alexandra Basolo. "It is not useful as a weapon. It is only used as a visual signal in a fight."

Benson, a researcher at Lynchburg College, says, "The sword might signal many things: how strong and vigorous he is, how well he ate while growing up or simply by indicating that he is large."

The researchers staged duels between swordtails (Xiphophorus helleri), by putting two comparably sized males with different-sized swords in a tank and then observing their interactions, which usually involved aggressive behaviour, such as chasing and biting.

The researchers then artificially manipulated sword size by affixing fake plastic swords to the fish.

Repeatedly, the male with the bigger sword won. Often all he had to do was to show his sword and then the other male slunk away.

"His body darkens, due to a stress response, he puts his head down and he then positions himself at the periphery of the tank," she says, adding that the loser even folds in his fins.

Dr Brian Trainor, a researcher in the College of Social and Behavioral Sciences at Ohio State University, says:

"One part I was a bit amazed with was that in the sword manipulation experiment; somehow the fish were able to determine their own artificial sword sizes.

"I think people usually think that fish are not too bright. Not only were these swordtails able to assess their competitors' apparent sword length, but also they were able to compare that with their own sword length. I thought that was a cool result."

He even believes there is a swordtail-human link, since both species seem to have evolved traits and behaviours that do not offer apparent survival benefits.

The sword, for example, appears to be useless, save for its visual impact. For humans, he says possible examples include artistic abilities and senses of humour.

Like male fish showing off their swords to females, he suggested that these human abilities might exist, in part, to impress members of the opposite sex.

Like the fish swords, they can also weed out competitors who may not be as talented or humourous.

"Our studies in swordtails show that sexually selected traits can be complex and used in multiple behavioural contexts, much like many aspects of human behaviour," he says.